The incidence of malignant melanoma and non-melanoma skin cancers has increased rapidly in Sweden during the last 20 years. The best-known way to revert this trend is primary prevention. Matching health messages to readiness to change in the population may enhance the effect of community-based prevention. The aims of this study were to investigate readiness to change sun-protective behaviour in two groups (visitors to mobile screening units and beach-goers) and to test a single-item algorithm in assessing the stage of change in sun-protective behaviour. Seven hundred and forty-two visitors to the mobile screening units and 202 individuals on nearby beaches answered a short questionnaire. The assessment of readiness to change was based on stages of change in sun-protective behaviour modified from the Transtheoretical Model of Behaviour Change. As expected, the visitors to the screening units were more often in action/maintenance stages than the beach group for most sun-protective behaviours. In conclusion, the single-item algorithm method appears to be sensitive to assess readiness to change sun-protective behaviour, based on the Transtheoretical Model of Behaviour Change. This method can be incorporated into population surveys and may aid in developing successful skin cancer prevention programmes.